1. Field of the Invention
This invention relates generally to a method for controlling a transmission clutch during a clutch control event.
2. Description of the Prior Art
A clutch transfer function is defined as a relationship between clutch torque transmitted through frictional interfaces and a clutch actuator control signal, which may be electric current to an electric clutch actuator, hydro-electric actuator pressure, clutch piston position or other variables.
Clutch torque is affected by various uncontrolled noise factors, such as actuator system variability and thermal sensitivity of hydrodynamic torque, during the clutch actuation process. For example, for a hydraulically-actuated wet clutch, clutch torque may be relatively linear with respect to a given control signal profile at a certain transmission fluid temperature. However, at a different transmission fluid temperature, clutch torque may exhibit significant nonlinearity and its value may be considerably different for the same given control signal.
The clutch transfer function varies from unit to unit due to hardware variability and also changes over the life of a vehicle due to degradation and wear of system components, including friction material, transmission fluid additives, hydraulic valves, etc. In practice, it is not easily possible to capture the changing transfer function behaviors in volume production applications using prior art technologies. Accordingly, a conventional clutch control methodology primarily relies on a clutch transfer function, which may be obtained a priori based on limited vehicle tests or bench tests.
The clutch transfer function may be adjusted based on indirect observations such as increased transmission shift duration. However, such an approach cannot directly and accurately map a detailed functional relationship between clutch torque and actuator control signal under all drive conditions.